A common and conventional bond for resin bonded grinding wheels employing the premium abrasives diamond and cubic boron nitride is phenol formaldehyde resin, usually containing fillers to add strength and improve grinding performance. Such wheels are normally of a composite construction in which a rim of abrasive grits in the resin matrix is adhered to a central, non-abrasive core, the core being adapted to be mounted concentrically on a shaft for rotation of the wheel. It is desirable that the core have relatively high thermal conductivity, to conduct heat from the grinding section, and good adhesion to the grinding section. In certain shapes, such as "cup-wheels" in which the diamond section is a ring bonded to the outer periphery of the larger end of a frusto conical ring, as in the drawing, it is desirable for the user to be able to "dress" the tool by removing portions of the core adjacent the grinding section, to avoid rubbing of the core material against the work being ground.
All these factors are conventionally satisfied by the use of a core material of phenolic resin, filled with particulate aluminum or other metal, to give good heat conductivity and strength, and desireably with graphite or other solid lubricant powder to improve the machining (or "dressing") properties. The mix for forming the core usually also contains lime or some other particulate filler necessary to prevent cracking of the core during post cure.
In the manufacturing operation the core is cold pressed and then the abrasive ring is applied while the core and ring are confined in a hot pressing mold. After the hot pressing operation the wheel is subjected to a post cure. This method avoids cracking problems which arise if it is attempted to hot press the abrasive ring on a completely cured core.
Unfortunately, the above described method introduces problems leading to cracking of the core because of absorption of moisture and loss of dimensional stability of the cold pressed cores if they are not immediately used for making wheels.
The present invention solves the cracking problem by eliminating the need for lime, while retaining the good bonding properties of phenolic resin in the core. Although useful for straight wheels it is of particular utility in more complex shaped cup wheels as in the present drawing.